Throughout this site we use
many technical terms, and given the breadth of readership
our site enjoys, sometimes we are remiss and incorrectly
assume everyone knows what we are referring to. For those
that do not, here are some explanations of the technical
terms use.
The return path for current flowing from the battery to any power-consuming item. Early vehicles with electrical devices had cables for both supply and return paths, but for many years on passenger cars the metal structure of chassis and body has formed one of the paths. This 'earth return system' is so called because the negative terminal of the battery is (except on some old cars) connected to the chassis.
Strip of metal, usually copper or brass, or a flat piece of wire braiding, used to connect an item such as a radio or the vehicle engine to a convenient earthing point on the metal structure or chassis. An earthing strap is frequently found between the engine or gearbox and the body shell.
Item which is not concentric about a given point or axis; for example, a circular disc which revolves about a point offset from its true centre. The term usually refers to a cam (which frequently in steam engine design was truly circular but rotating eccentrically) which gives a means of converting rotary motion to linear motion.
Air valve, usually operated automatically by induction manifold depression, which on fixed-choke carburettors gives an air bleed into the delivery ducts to permit more economical consumption when the engine is cruising.
Current set up momentarily in a mass of metal when it is situated in the vicinity of a charging current. The phenomenon occurs in the iron core of an electro-magnet and, as part of the energy is dispersed as heat, steps are taken to reduce the eddy currents.
Type of auxiliary brake or 'retarder' used on commercial vehicles to provide additional braking and safety on long descents. On brakes of this type, one or both faces of an iron disc rotate in close proximity to the stationary poles of an electro-magnet. Current supplied from the battery causes a magnetic flux, giving progressive braking as the current is increased.
Mean, or average, pressure of the varying values existing in the combustion chamber throughout the power stroke of the piston. This can be used to calculate the average gas force exerted on the piston throughout the stroke.
The ratio, usually expressed as a percentage, of actual achievement to a theoretical possible maximum. Thus, brake efficiency is the ratio of actual retardation (deceleration) to the acceleration due to gravity (ie 32.2ft per second per second). For example 80% braking is approximately 26ft per second.
Polymer having large molecules with a repeating pattern of atoms and possessing good elasticity. Polymers give rubber-like properties to some plastic or synthetic materials
Auxiliary brake fitted to commercial and public service vehicles to give additional and smoother braking and to reduce wear and tear on the normal friction lining brakes. For relative simplicity and low cost, the electric brake is generally of the eddy current type.
Welding process much used in vehicle construction and body repairs, in which an electric current of high amperage melts both the metal base and the welding rod by striking an arc between them. The process can be used by DIY mechanics, but requires stringent precautions to be taken to avoid damage to the eyes.
Chemical reaction which produces an electric current or conversely where a current flowing through matter causes a chemical reaction. Thus the charging and discharging of a lead-acid battery are examples of a reversible electrochemical process.
Conductor by which current enters or leaves a device, such as a thermionic valve or electrolytic cell. Spark-plugs have a central electrode and an earth electrode.
Term originally used by Faraday for those liquids which would carry a current, for example water to which a small amount of acid or salt has been added. It is now used for the liquid in electrolysis and, hence, for the solution in batteries, for example diluted sulphuric acid in the lead-acid battery.
Device producing magnetism from any electric current. Electricity is passed through a coil surrounding a core of laminated soft iron, causing it to become temporarily a bar magnet with north and south poles dependent on the direction of the current. Such devices are used in motor vehicles for solenoid switches, pre-engaged starter motors, overdrive control valves and so on.
The generation of electricity through the agency of magnetic lines of force - that is a magnetic field. All dynamos, alternators and magnets are based on this principle.
Negatively-charged particle of an atom. Atoms have a positively-charged nucleus of protons surrounded by smaller, negatively-charged units. Electronic theory is that these electrons are responsible for the flow of electricity, and that good conductors possess free electrons in constant motion between the atoms, so that the application of an electromotive force causes them to move in a definite direction.
Injection system in which an electronic control determines the quantity of fuel injected into the cylinders or induction manifold to accord with power requirements and other influences such as atmospheric temperature and pressure.
Ignition system in which a transistorized unit makes and breaks the primary current to the coil. In some, the orthodox contact breaker in the distributor is retained, but merely interrupts a small non-inductive current in the transistor base circuit. The resulting absence of arcing at the points obviates burning and pitting, and hence greatly lengthens the life of the points, as well as improving low-speed performance and giving more accurate timing. On a fully transistorized system, the contact breaker is replaced by either a photocell and interrupted light source, or an electro-magnetic trigger.
Abrasive material consisting of a coarse fabric to one or both sides of which are cemented granules of carborundum (emery). Emery cloth is available in various grades of coarseness and as strip or sheets, and is used for surface finishing and polishing.
Grinding wheel manufactured from carborundum grains, coarse or fine. It is rotated at speed either by an electric motor or by a hand-driven geared machine, to permit the grinding of metals (normally only ferrous metals, as soft metals clog the surface), and the sharpening of drills and other tools. Care must be taken to avoid overheating of the item being ground, which would cause loss of the metal's temper and burning.
In some fixed-choke carburettors, a ducted die-casting which mixes petrol and air to form a spray. In another make of fixed-choke carburettor, an emulsion tube serves the same purpose.
The full possible movement in an axial direction of any shaft, hub, or other rotating item, as limited by its bearings or relating devices. For example, the end float of a crankshaft is limited by shoulders which bear against the mating surfaces of one or two of the main bearings.
The unburnt gases in the combustion chamber, ahead of the flame front after ignition and not yet ignited. Due to excess pressure and temperature, the end gas may explode to cause detonation (pinking).
Any machine (for example, engine of war) but, in the vehicle sense, the device producing the power to propel the vehicle, by converting heat energy from the internal combustion of fuel into mechanical energy.
As the orthodox internal combustion engine is based on reciprocating parts, these would produce oscillating reactions and transmit vibrations to the car structure unless they were in some way counterbalanced�for example, by crank counterweights, horizontally opposed pistons, or countershafts. It is impossible perfectly to balance a single cylinder engine, but multi-cylinder arrangements make an acceptable approximation possible.
The use of the engine while in a low gear to provide retardation on descending a hill, due to the energy required to turn the engine against compression and with the throttle closed. On some diesel engines this effect is greatly increased by providing a butterfly to close the exhaust outlet.
A means of venting the crankcase to prevent the build-up of gases escaping past the piston rings, and also water vapour, from causing a rise in pressure which might force oil past tlie main bearing seals and into the clutch housing. Formerly this was effected by providing an external pipe from near the top of the crankcase but now the crankcase is usually vented into the carburettor intake as a means of reducing atmospheric pollution.
The total swept volume of all the cylinders, that is, the volume of the cylinder between TDC and BDC and thus excluding the clearance volume of the combustion chamber. In Europe, engine displacement is usually given in litres or in cubic centimetres; in North America, in cubic inches.
Bracket supporting the engine on the chassis or engine-bay framework. Since the introduction of flexible engine mounting by Citroen in the early 1930s, engine mountings on all but very large commercial vehicles include rubber cushioning of the Metalistik-type.
In the opposite direction to the torque generated on the crankshaft, there is an equal reaction on the crankcase. If the crankshaft is turning clockwise as viewed from the front, the crankcase is attempting with equal torque to rotate anticlockwise. Because the crankcase is attached to the structure of the car, which absorbs the torque, engine torque reaction is less obvious on a car than on, say, a motorcycle with an in-line crankshaft.
Reduction gearing some of whose gear wheels rotate on axes which also are rotating around the axis of a meshing gear wheel. Most systems consist of a driven sun wheel and an annulus wheel, between which are meshed two or three planet wheels mounted on a spider on an output shaft. By arresting the annulus by a friction brake or magnetic brake, the sun wheel is caused to drive the planet system in the same direction at a reduced speed, depending on the relative sizes of annulus and sun wheel.
Casing of a Wankel-type, planetary-rotation engine, in which the bore or interior wall shape is an epitrochoid curve - a curve generated by radii from a rotating centre.
One of a number of synthetic polymers (chemical components of a special type) used as structural plastics, surface coatings and adhesives. Epoxy resin is also used extensively for enclosing small electronic devices into one component or capsule.
The base (C2H5) of many hydrocarbon compounds such as alcohol, ether and ethylene. It is also involved in tetra-ethyl-lead, a petrol additive which improves a fuel's "anti-knock" abilities.
Additive to the water in cooling systems to prevent freezing and consequent damage to engine castings and radiator. It lowers the freezing point progressively and produces a mush of frozen granules before finally solidifying, thus giving added protection against bursting. Proportions to be used to give adequate protection are advised on the container. "Glycol" needs inhibitors to reduce the possibility of corrosion and must be treated with care as spillage will ruin paintwork unless washed off immediately.
Petro-chemical material used as the basis of synthetic rubbers. Such rubbers have excellent mechanical properties but poor resistance to organic solvents.
System designed to prevent the escape of petrol vapour from the fuel tank and carburettor whether the engine is running or not. The tank is sealed by a pressure-vacuum valve and an air-tight filler cap, so allowing air to enter only as fuel is consumed. The tank is vented to the carburettor via a fuel trap and a large canister containing activated charcoal. When the engine is not running, any vapour from the tank is trapped by the charcoal. When it is, filtered air is drawn through the canister, purges the charcoal, and the vapours are burnt in the engine.
In most types of air conditioning system the first stage of the system. It uses a cooled metal plate or grid to cool the incoming air and cause condensation on the faces of the plate, the moisture then serving to pick up the dust content which drains away with the water. The air is thus reduced in humidity and cleaned before further treatment.
Device for determining the relative amounts of carbon dioxide, oxygen and carbon monoxide present in exhausts. There are various types of analyzer. One simply measures the thermal conductivity of the gases and recording on a galvanometer whether the fuel-air mixture is rich (excess carbon monoxide) or lean (excess oxygen). Another type involves an endiometer where the exhaust gases are bubbled through a succession of chemical solutions which respectively absorb carbon dioxide, oxygen and carbon monoxide. More sophisticated types can ascertain the content of pollutants.
Usually a robust iron casting, but sometimes fabricated from special sheet steel, a device which is attached to the cylinder head to collect exhaust gases from each of the exhaust ports and direct them into the exhaust pipe.
Expansion chamber in the form of a canister interposed into the exhaust system. It may be a fairly simple box with inlet and outlet pipes protruding inside so that the change of direction of gas flow and change of pressure provides some silencing effect. Usually the box is fitted with baffle plates to cause further changes in direction and sometimes, in the interest of reducing back pressure, the exhaust pipe runs straight through but has many holes drilled in it and has a surrounding packing of glass-fibre or asbestos for sound deadening. On some cars several silencers of differing types are used with the object of maximum silencing with minimum back pressure.
Term used for all components�pipes, expansion boxes and silencers - of the exhaust system; thus "exhaust fracture" or "blown exhaust". The same term is used for the gases exuded from the engine cylinders, including water vapour and other products of combustion, unburnt carbon, carbon monoxide and various pollutants; thus "exhaust fumes". A modern car's exhaust system consists of an exhaust manifold, pipes and a muffler or mufflers. The manifold is responsible for extracting the burnt gases from the combustion chamber as quickly and completely as possible. The manifold fitted to most production cars is, like many other components, a compromise. The cast iron manifold commonly used is quite capable of doing the job, and is cheap and easy to manufacture. But it does produce unwanted turbulence in the combustion chamber which hinders the next intake of fuel and air. An extractor manifold cures this problem by taking the gases away from the exhaust.
Turbine driven by the flow of exhaust gases and having mounted on the same shaft a compressor ("blower") which is used to boost the air-fuel intake of a four-stroke engine (or to charge the cylinders of a two-stroke engine). The turbocharger may be operating constantly or only brought into play at higher throttle settings for accelerating and higher speeds. Superchargers were formerly used only on racing car engines, or on aircraft piston engines to compensate for loss of air density, and hence power, with increasing altitude. They were mechanically driven from the crankshaft through gearing. Now, exhaust turbochargers are being offered on the more sporting saloon cars, even with relatively small engines. The compressor is usually of centrifugal vane type (somewhat similar to the fan in a vacuum cleaner) and the energy to drive it is derived from some of the heat energy which would be lost in a normal exhaust flow.
Valve in a combustion chamber which is opened to allow the exit of exhaust gases. It thus operates at very high temperatures and has to be of suitable alloy steel to avoid distortion, loss of mechanical properties and surface flaking. If the inlet and exhaust valves are of different sizes, the exhaust valve is the smaller and sometimes its seating angle differs from that of the inlet valve. Some exhaust valves have a tough, welded-on seating area and in some racing or aero engines the valve stem is hollowed to contain sodium which, on melting, oscillates and improves heat conduction.
The dimension of the expansion of a unit length of metal when its temperature is raised by one degree of temperature. Thus, for a steel, the linear coefficient of expansion may be 0.12 x 10-4 (ie 0.000013) per �C. The coefficient of cubical expansion is taken as three times the linear value.
Sheet metal convex disc fitted into a machined core-hole of a cylinder block or head and expanded by flattening to seal the hole securely. Expansion plugs sometimes perforate through corrosion but, if accessible, can be easily replaced. The original holes are necessary for removal of the core moulds after the block is cast.
Term usually referring to the tank fitted on certain cars with sealed cooling systems. On expansion of the coolant on heating, a pressure valve opens (often that in the filler cap) and surplus coolant flowing out is piped to the expansion tank. On cooling, the pressure in the system falls as the coolant contracts and fluid from the tank is syphoned back through a second valve into the main system. On certain vehicles where, because of the frontal shape, the radiator has to be sited too low to permit it to have an air space above the level of liquid, an expansion tank is fitted at a suitable higher sitting and the pressure filler cap is found on top of this tank.
In an air conditioning system a pressure-operated valve placed between the condensor, where the refrigerant is at a high pressure, and the evaporator. It reduces the pressure and so causes evaporation; so heat is taken from the air circulating through the heat exchanger in the evaporator. A temperature-sensing bulb in the evaporator controls the metering of refrigerant through the expansion valve.
Term sometimes used to describe the antechamber (swirl chamber, pre-combustion chamber) on compression - ignition ("diesel") engines of "indirect injection" design. Fuel is injected either entirely or principally into this chamber. It ignites and, while burning, is forced through a small passage into the main combustion chamber. This design gives somewhat lower maximum ignition pressures and thus permits lighter construction of the engine as well as smoother running and is thus popular for smaller commercial vehicles and private cars.
Oil specially compounded with additives to produce a tenacious film, preventing the oil from being squeezed out between surfaces under high loading. It is used on, for example, gear teeth, particularly in the crown wheel and pinion.
Bolt with a specially formed head to provide a ring or a flat lug with a transverse hole to which may be fitted a fork end by means of a clevis pin or bolt.
